FIG. 1 shows a simplified block diagram of a known, direct-mixing receiving device. This direct-mixing receiving device 100 has an analog receiver section and a digital receiver section. In the analog receiver section, an antenna 10 for receiving an analog, digitally modulated received signal is arranged at the input end. At the output end of the antenna 10, the received signal is supplied to a low-noise amplifier (LNA) 20. The received signal amplified by the low-noise amplifier 20 is supplied to a mixer 30 by means of which the received signal is mixed into the baseband. For this purpose, the mixer 30 is connected to a local oscillator 40 by means of which an oscillation having a frequency which corresponds to the carrier frequency of the received signal is generated and supplied to the mixer 30 in the form of two mutually orthogonal oscillations. The received signal is thus split by the mixer 30 into an I and a Q signal component. On the I path, the I component of the signal is first supplied to a first low-pass filter 51. The output signal of the first low-pass filter 51 is supplied to a first analog-controlled amplifier (VI) 61. The output signal of the first amplifier 61 is supplied to a first analog/digital converter 71. The output signal of the first analog/digital converter 71 is supplied to the digital receiving section 80. The Q component of the output signal of the mixer 30 is supplied to a second low-pass filter 52. The output signal of the second low-pass filter 52 is supplied to a second analog-controlled amplifier (VQ) 62. The output signal of the second amplifier 62 is supplied to a second analog/digital converter. The output signal of the second analog/digital converter 72 is also supplied to the digital receiving section 80.
For the receiving device, an analog gain control or level control (AGC, automatic gain control) is carried out. For this purpose, a received signal level is determined at a suitable place on the entire received-signal path, for instance by an RSSI measurement, and from this received signal level a control signal is generated. This control signal is supplied as analog level adjustment signal to the amplifiers 61 and 62.
The quality of the receiving characteristics of a direct-mixing amplifier is significantly dependent on the achievable symmetry between the I and Q path. A special problem is here the synchronism of the controllable amplifiers 61 and 62. As shown in FIG. 1, analog-controllable amplifiers 61 and 62 are used in the prior art, on the one hand. With increasing use of CMOS technology instead of bipolar technology as the basic circuit technology, however, the use of analog-controlled amplifiers becomes more and more expensive since the CMOS transistors cannot be produced with constantly good quality features with respect to one another to an adequate degree. This makes it difficult to achieve a synchronism of the amplifiers.
A further approach to a solution of the prior art is the use of digitally adjusted control amplifiers. In general, this results in very high requirements for the quantization of the digital corrupting variable (e.g., OFDM/QAM64 for DVB-T/H), depending on the type of modulation. If required or desired, the digital level control can be effected with a resolution within a range of tenths of dB. However, increased implementation expenditure is a disadvantage with digital level control.